The field of application of the invention is more particularly that of thermostructural composite materials, i.e. composite materials having mechanical properties that make them suitable for constituting structural elements and that are capable of conserving those properties up to high temperatures in use. Typical thermostructural composite materials are carbon-carbon (C-C) composites and carbon matrix composites (CMCs) in which the fiber reinforcement comprises refractory fibers made of carbon or of ceramic.
The manufacture of a part made of thermostructural composite material generally comprises making a fiber preform whose shape is close to that of the part that is to be manufactured, and then densifying the preform with the matrix.
The fiber preform constitutes the reinforcement of the part and its function is essential for the mechanical properties thereof. The preform is obtained from fiber fabrics: threads, cordage, braids, pieces of cloth, felts, etc. . . . Shaping is performed by winding, weaving, stacking, and optionally needling together two-dimensional plies of cloth or sheets of cables, etc. . . .
The fiber preform is densified by filling the voids throughout the volume of the preform with the material that constitutes the matrix.
A first densification technique (liquid process) consists in impregnating the preform with a liquid composition that contains a precursor of the matrix material and then, possibly after drying and polymerization, in subjecting the impregnated preform to some kind of treatment, generally heat treatment, for transforming the precursor. Several consecutive cycles going from impregnation to heat treatment are usually required to achieve the desired degree of densification.
A second densification technique (gas process) consists in performing chemical vapor infiltration of the matrix-constituting material into the preform. To this end, the preform is placed in an infiltration oven into which a gas is admitted. Under determined conditions of temperature and pressure, the gas penetrates to the core of the preform and, on making contact with the fibers, the matrix-constituting material is formed by decomposition of the gas or by a reaction between components of the gas.
To keep the preform in the desired shape while chemical vapor infiltration is taking place, it is common practice, at least during a first portion of the densification process, to hold the preform in tooling that is generally made of graphite. Such bulky tooling is expensive to make, particularly when the preform is complex in shape. It also requires a large number of holes to be machined so as to give the gas access to the preform through the tooling. In addition, the tooling is fragile, heavy, and cumbersome.
Now, chemical vapor infiltration is a process that is often lengthy and expensive. The presence of tooling occupying a significant fraction of the working volume of the infiltration oven and presenting a large amount of thermal inertia is penalizing. In addition, matrix material is inevitably deposited on the tooling, thereby giving rise to a non-negligible quantity of rejects due to adhesion between the preform and the tooling and, in any event, to changes of dimensions that prevent the tooling being reused directly. Thus, under the best of circumstances, such deposits require the tooling to be renovated frequently.
The presence of tooling during chemical vapor infiltration is necessary only until the preform has been consolidated. This stage is reached when the material constituting the matrix has been deposited in sufficient quantity to bind together the fibers throughout the volume of the preform so that, after the tooling has been removed, the preform remains in the desired shape and can be handled. Densification of the preform can then be finished off without tooling. Nevertheless, tooling remains necessary at least during a part of the infiltration process, and, after consolidation, the process must be interrupted in order to enable the tooling to be withdrawn.
It is therefore desirable to be able to perform the entire chemical vapor infiltration process without it being necessary to hold the preform in tooling.
To this end, U.S. Pat. NO. 08/013,816 of Feb. 2, 1993, now U.S. Pat. No. 5,336,522, proposes consolidating the preform by a liquid process. The fiber fabric constituting the preform is impregnated by a precursor of the matrix, and is then shaped by means of tooling (mold or shaper). Thereafter, the precursor, possibly after drying and polymerization, is transformed by heat treatment so as to leave behind a solid residue that serves to consolidate the preform.
For thermostructural composite materials, and in particular CMCs, a considerable improvement in resistance to shocks and to cracking is obtained by forming an appropriate matching layer or "interphase" on the fibers of the fiber reinforcement prior to densification. The interphase-constituting material is selected to change the propagation mode of cracks so that cracks propagating through the matrix do not reach the fibers and pass through them thereby degrading the mechanical properties of the material. Reference may be made, for example, to U.S. Pat. No. 5,026,604 which describes the use of chemical vapor infiltration to form interphases made of pyrolytic carbon or of boron nitride.
When the preform is being consolidated by a liquid process in order to avoid the need for tooling during chemical vapor infiltration, it is necessary either to form the interphase on the fiber texture while it is not being held in shape by tooling, or else to use fibers that have previously been coated with an interphase layer prior to making the preform.
However, the presence of the interphase stiffens the fiber fabric, thus giving rise to difficulties in shaping the preform, particularly if the shape of the preform is complex. In order to conserve sufficient capacity for deformation, it is necessary to limit the thickness of the interphase, thereby increasing the risk of the interphase being damaged by deformation of the fabric. However, if the interphase is to perform its function effectively, it is essential for it to cover the fibers of the preform in a manner that is continuous and uniform.